Extremes and predictability in the European pre-industrial climate of a regional climate model

Abstract

A high-resolution pre-industrial control simulation with the regional climate model REMO is analyzed in detail for different European subregions. To our knowledge, this is the first long pre-industrial control simulation by a regional climate model as well as at comparable resolution. We assess the ability of the climate model to reproduce the observed climate variability in various parts of the continent. In order to investigate the representation of extreme events in the model under pre-industrial greenhouse gas concentrations, selected seasons are examined with regard to the atmospheric circulation and other climatic characteristics that have contributed to the occurrences. A special focus is dedicated to land-atmosphere interactions. Extreme seasons are simulated by the model under various circumstances, some of them strongly resemble observed periods of extraordinary conditions like the summer 2003 or autumn 2006 in parts of Europe. The regional perspective turns out to be of importance when analyzing events that are constituted by meso-scale atmospheric dynamics. Moreover, the predictability of the European climate on seasonal to decadal time scales is examined by relating the statistics of surface variables to large-scale modes of variability impacting the North Atlantic sector like the Meridional Overturning Circulation, the El Niño Southern Oscillation, and the North Atlantic Oscillation. For this purpose, we introduce a measure of tail dependence that quantifies the correlation between extreme values in two variables that describe the state of the climate system. Significant dependence of extreme events can be detected in various situations.

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Acknowledgments

We would like to thank Bjorn Stevens, Monika Esch, Michael Botzet, Wolfgang Müller, Johann Jungclaus, Philip Lorenz and Alberto Elizalde for support and helpful discussions.

Appendix Atmospheric characteristics of extreme events

In the following we describe the atmospheric characteristics of the extreme events referred to in Sect. 3.

Temperature extremes

Summer 2434

Very high temperatures occur in Central Europe, but also in the Alps, France, the Mediterranean, Eastern Europe, and on the Iberian Peninsula. A high-pressure system is centered over Central Europe, a low-pressure through North East of Scandinavia. Seasonal mean winds are from the South West to the Iberian Peninsula, the Alps, and Central Europe.

For Central Europe, the Alps, France, and the Iberian Peninsula highest temperatures take place in July. The high-pressure system over Central Europe remains stagnant over the season. The pressure gradients get somewhat flatter in August and the core of the high is moving slightly to the East. Accordingly, highest temperatures for Eastern Europe and the Mediterranean occur in August.

Negative cloud cover anomalies are obvious all over the continent except Scandinavia. The main signal covers Central Europe, the Alps, France, the Mediterranean, and Eastern Europe. Incoming solar radiation and surface thermal radiation anomalies match broadly with cloud cover anomalies. Latent heat flux anomalies are positive over the season in Central Europe, France, the Iberian Peninsula and most of Scandinavia, but negative in parts of the Mediterranean and Eastern Europe, indicating a drying of the soil in these areas.

Autumn 2351

Especially warm conditions prevail on the British Islands, but high temperatures are attained also in the Alps, Central Europe, France, and Scandinavia. The high-pressure system centered over the southern Baltic Sea attracts warm air from the South to the British Islands, the Alps, and Central Europe. Low pressure establishes itself over Iceland. Eastern Europe experiences winds from the North.

In all regions, temperatures are highest in September and lowest in November, but extraordinary warm conditions prevail also in October. In September the high is centered over Scandinavia, extending to the British Islands, Central Europe, the Mediterranean, and Eastern Europe, bringing warm air on the western side to the British Islands and Central Europe. In October and November the atmospheric condition is very stable.

The negative cloud cover anomaly over the British Islands is not particularly pronounced suggesting that the anomalously high temperatures are mainly due to the advection of southerly warm air. Latent heat flux anomalies are positive in parts of the British Islands, France, and the Iberian Peninsula, but negative values dominate on the rest of the continent.

Spring 2257

Very warm conditions dominate in the Mediterranean, but also on the Iberian Peninsula, in Eastern Europe, and the Alps, to a lesser extent in France, Central Europe, and Scandinavia. High pressure extends over Eastern Europe, and the wind transports warm air from the South to the Alps and the Mediterranean.

High temperatures already affect the Mediterranean and the Alps in April, but the warming even increases in May. The atmospheric state is not extraordinarily stable. In March a high-pressure system is located over Central Europe, moving to the Mediterranean in April. A low establishes west of the European continent in May, high pressure extends from Iceland to Eastern Europe, causing southerly airflow to the Mediterranean. The reasons for warm temperatures in the Mediterranean are different in April and May. The advection of warm air from the south plays an important role, the very warm spring in the Mediterranean is a dynamical phenomenon rather than provoked by stable conditions.

Less cloud cover than average can be asserted mainly in the Mediterranean and Eastern Europe. The distinct negative latent heat flux anomalies are primarily restricted to the Mediterranean and the Iberian Peninsula.

Summer 2180

Extraordinary high summer temperatures strike Eastern Europe, very warm conditions also prevail in Central Europe, the Alps, France, and the Mediterranean. A very strong and persistent high-pressure system remains stable and centered over Eastern Europe, covering the whole continent. Winds from the North West keep temperatures moderate on the British Islands.

The Alps and Central Europe experience warmest temperatures in July, Eastern Europe and the Mediterranean in August. Strong high pressure with centre over France can be observed in June (when France experiences highest temperatures), moving to the East in July. Warm air from the south is transported to the Mediterranean. The high is moving further east in August, when a low-pressure system develops northwest of the British Islands. The atmospheric circulation shows little variation, especially in Eastern Europe and the Mediterranean, where a strong blocking pattern prevails.

Very distinct negative cloud cover anomalies extend all over Europe, with the exception of Scandinavia and the British Islands. Strongest anomalies affect Eastern Europe, where the heat wave is most pronounced. Latent heat flux anomalies are negative in the Mediterranean, Eastern Europe, Central Europe, and Scandinavia, positive on the Iberian Peninsula, parts of France and the British Islands.

Precipitation extremes

Autumn 2228

Very high precipitation occurs in the Alps and the Iberian Peninsula, but also in France, the Mediterranean, and Eastern Europe. A strong cyclone is centered over the Alps, while Scandinavia and the British Islands are on the edge of this low-pressure system. Westerly airflow dominates over the continent.

In the Alps and France strongest precipitations fall in September, least in October. On the Iberian Peninsula rainfall increases over the three autumn months. Eastern Europe sees strongest precipitation in October. The low-pressure system remains stable over the Alps. It is less extended in September, somewhat weaker in October, but stretching more to the East.

Positive cloud cover anomalies cover the Alps, France, the Iberian Peninsula, the Mediterranean, and Eastern Europe. Latent heat fluxes are above average over most of Europe except on the British Islands and parts of Central Europe and Scandinavia.

Winter 2362

Very high precipitation anomalies affect France, but also Central Europe, and the Alps. An extended low-pressure system is centered over Europe with westerly flow conditions.

For France, Central Europe, and the Alps strongest precipitations occur in January, but rainfall is distinctly above average also in December. Pressure conditions are very stationary over the three months. Westerly windflow impacts the three regions with highest precipitation anomalies.

Positive cloud cover anomalies can be observed in a band from the Iberian Peninsula to France, Central Europe, and the Alps, correlating clearly with the precipitation anomalies. Correspondingly, a strong positive anomaly in latent heat fluxes affects France and the regions of high rainfall.

Spring 2343

The British Islands receive an anomalous amount of precipitation, but the rainfall is high also in Scandinavia. In the Mediterranean the amount of precipitation is below average. The low-pressure belt extends from North West of the British Islands, over the British Islands and towards Scandinavia. The cyclonic structure of the flow leads to west winds over the British Islands.

On the British Islands and Scandinavia highest precipitation falls in March. The pressure system is not particularly stable. In March the atmospheric state is similar to the seasonal mean situation. In May, a low-pressure ridge extends from the British Islands southeastwards to Eastern Europe, while high pressure is located over Scandinavia. In April a low-pressure band stretches over Northern Europe. From the continental point of view, the situation is quite dynamic, but the British Islands remain influenced by northeasterly airflow and low-pressure conditions throughout the season.

Positive cloud cover anomalies reside over the British Islands, Southern Scandinavia, and Northern Central Europe. A positive signal in latent heat fluxes can be observed on the British Islands, in the Alps, and parts of Scandinavia.

Winter 2317

Very high precipitation can be observed in the Mediterranean, rather low in Central Europe, British Islands, and Scandinavia. A strong low-pressure system remains stationary over the whole season in the Mediterranean, and high pressure over Scandinavia. A cyclonic structure of the wind can be observed over the Mediterranean. The NAO is in a distinct negative phase.

Precipitation over the Mediterranean is highest in January, but strong also in December. According to the pressure pattern, cloud cover anomalies restrict mainly to the Mediterranean. No distinct signal is apparent in other regions. Latent heat flux anomalies are strongly positive over almost the whole Mediterranean Sea, but negative over the British Islands and Central Europe.